Efficient empty volume rendering in Cycles
It's actually quite similar in Cycles and seems to speed up renderings as well (which indicates there is still room for optimization in Cycles volume rendering).

So with a basic setup:


You get result (~16 seconds):


Now add a Shader Mix Node and use Density as a Mix factor:


With current Cycles algorithms you can leave the second slot unconnected - it just tells Cycles to compute volume rays for areas that are masked by density with a value greater than 0.5 (screen shows the reverse of that, but it's the same if you connect Volume shader in second slot and use converter math node 'Greather than', which would be more intuitive, I guess).

Now the result of this little tweak:

Time almost halves! But there are very crisp edges - you can tweak that with changing the converter math node value to a very low number (which would tell cycles to leave out only area with density values close to zero:


And the result:


Still reasonable improvement over the original render time and nearly the same visual effect.